Prime Minister John Key says Kim Dotcom will never prove he's lied - but has challenged him to produce his evidence.

He believes Dotcom is referring to an August 2011 visit to his electorate office by two tradesmen who were throwing their weight behind the German's bid to buy his Coatesville mansion.

The approach was handled by his agent and passed to Land Information Minister Maurice Williamson - but never referred to Key, he said.

"Lying is that you knowingly and deceitfully mislead people. That'll be impossible for Kim Dotcom to prove. I'll tell you that for nothing because I have no recollection of hearing about the guy before the 19th of January.

"So if he is miraculously going to find someone who he reckons whispered in my ear something, well OK. Prove the person and put them up."

He stressed: "I've never lied about Kim Dotcom and I don't lie about anything. I don't always give you the answer you might want to hear because that's prudent. But I don't lie about things."

Key has been fending off questions about the extradition case for months and the Megaupload mogul was even raised during an informal conversation with US president Barack Obama in Camodia yesterday.

Dotcom said this week he can prove Key isn't telling the truth. He believes there was undue political influence in the process to extradite him to face copyright infringement, racketeering and money laundering charges.

"What difference would it make if I'd heard about the guy six months earlier...it's not in my interests?" Key said.

"I have utterly rejected those conspiracy theories. In the fullness of time I'm very confident that when the case is fully laid out my position will be affirmed.

"His great conspiracy that we are puppets of the Americans is all wrong."

Dotcom is back in court tomorrow as his legal team try to obtain more details about illegal surveillance carried out by the Government Communications Security Bureau.